Galvanic corrosion is a type of electro-chemical corrosion that happens when two different metals, or alloys, are in contact with each other in the presence of an electrolyte (such as water with salts or other ions).
Essentially, one metal corrodes faster than it normally would, while the other corrodes slower (or is protected).
It is not hard to find stark warnings online that you should not use stainless steel brackets with cast iron because it may lead to galvanic corrosion. The irony of such statements is not lost on us with these warnings being issued by companies that are selling much lower quality products which themselves are considerably more prone to rusting. In fact mild steel brackets are just as likely to result in galvanic corrosion as any other metals, so saying that you shouldn’t stainless steel for your brackets because of potential rusting is like saying that you shouldn't use cast iron guttering in the first place.
It is a well-known fact that 316L stainless steel brackets are more durable, stronger and resistant to rusting than any mild steel alternative. They do not rely on an extremely thin coating of zinc to prevent rusting and are much easier to handle during transport and installation. The only thing stopping a zinc plated bracket from rusting is the coating so any tiny scratches or dents and you will considerably shorten the lifespan of those brackets.
Now when it comes to galvanic corrosion it is advised that the two metals do not come into direct contact with each other. So what should you do to stop your gutter brackets causing galvanic corrosion? Firstly it would be highly usual for any cast iron rainwater system not to have a protective paint coating that included both a primer undercoat and top coat. In fact, due to the exceptional lifespan of cast iron rainwater systems, it is often the case that they have decades of paint on them. Provided that there are no exposed areas, particularly where the gutter brackets are situated, then this will provide a barrier between the metals.
One of the benefits of 316L stainless steel is that is does not require a protective coating to stop it rusting. That said, many customers prefer their brackets to match the colours of the cast iron rainwater system that they are used on and applying paint to the brackets will only further protect against galvanic corrosion. We would advise the application of a non-conductive coatings (epoxy paint, powder coating) to either surface to prevent metal-to-metal contact and importantly regularly maintain/paint your cast iron rainwater system, treating any exposed surfaces to break the corrosion circuit.
Whilst suppliers of plated metal brackets will try and have you believe that stainless steel gutter brackets will cause you significant issues, a well maintained rainwater system with stainless steel brackets is at no more risk of galvanic corrosion than one with mild steel brackets. It is not usual for the mild steel alternatives to be imported from countries such as China and India. This is because of price and not quality and there is a good reason why Metal Gutter Brackets UK use a significantly higher quality material and manufacture in this country.
Next time you read or are told that stainless steel brackets should not be used with cast iron guttering, it is worth remembering the alternative that you are being offered has a very thin zinc coating typically being just 5–25µm (0.005–0.025 mm) thick and whilst it does provide a much smoother finish than galvanising, it offers a relatively low corrosion resistance. In fact zinc plated products would typically be used for indoor or light-duty applications, so telling people that this as a viable alternative to 316L stainless steel because stainless might cause rust is just a lazy scare tactics. You are far more likely to have a plated mild steel bracket rust on its own than you are to have a 316L stainless steel bracket cause rust through galvanic corrosion.